This application relates to a degradable agricultural mulch film which includes slowly released multi-nutrients in a matrix of thermoplastic resin material. The application relates more particularly to a multilayer mulch film in which the release of the multinutrients is time controlled to provide the optimal amount of nutrient at the various stages of plant growth. The application also relates to a method for preparing the mulch film disclosed herein.
Agricultural much films are used extensively throughout the world because of the many advantages they provide, including retarded weed growth, increased soil retention of moisture and heat, and reduced soil erosion by winds and rain. Such mulching films also enhance soil structure by preventing soil crusting and soil compaction.
In our co-pending U.S. patent application Ser. No. 792,656, now U.S. Pat. No. 4,686,790, Aug. 18, 1987 we disclose a plastic mulchc film which has good mechanical properties, clarity, the ability to retard weed growth without the use of herbicides or black pigments, degradability, the incorporation of nutritional materials, slow release characteristics and which is safe for the environment and ecology because it does not include toxic chemicals. That film disintegrates over the course of a growing season and releases the necessary nutritional value to plants in a controlled manner.
In essence, the plastic mulch film disclosed in the aforementioned application includes first film formed from a water-soluble synthetic resin such as a polyvinyl alcohol having admixed therein at least one releasable form of nitrogen and at least one releasable plant nutrient in addition to nitrogen and, a second film is formed as an adherent coating on the first film. The second film comprises a water-resistant resin such as a polyvinyl acetate which functions to retard the degradation of the mulch and thereby slows the rate of release of the nutrients.
The use of slow release nutritional mulch films as described above have resulted in earlier crop harvest, better vegative growth, increased yield, reduced weed growth, and less temperature build up. However, it has been found that such films do not provide the optimal amount of nutrients during the various stages of plant growth.
For example, Hunter Johnson, Jr. and Gary W. Hickman reported in a publication of the University of California, leaflet 2775 on Greenhouse Cucumber Production that cucumbers require only part of the total nitrogen (N) needed and most of the needed phosphorous (P.sub.2 O.sub.5) and potassium (K.sub.2 O) in the preemergent or preplanting stage. Specifically, they reported that during the preemergent stage, cucumbers require 5.7 grams per square meter (gm/m.sup.2) nitrogen (N), 17 gm/m.sup.2 phosphorous (P.sub.2 O.sub.5) and 28 gm/m.sup.2 potassium (K.sub.2 O). Then during the first few weeks of growth and prior to fruiting approximately 3.4 gm/m.sup.2 of nitrogen is needed. And then, during the weeks of fruiting, approximately 25.0 gm/m.sup.2 of nitrogen is needed.
Similar studies indicate that different crops require different nutrient doses at different stages of their growth cycle. For example, the following schedule for fertilization have been suggested:
Wittwer and Honma (1979) reported a fertilizer program for greenhouse early spring tomatoes that received preplanting and supplement fertilizer applications. The program is represented in the following table.
______________________________________ gm/m.sup.2 Stages of growth N P.sub.2 O.sub.5 K.sub.2 O ______________________________________ Preplanting (manure) 113.6 56.8 113.6 0-113 days 31.7 50.2 66.4 113-147 days 35.1 -- 45.0 ______________________________________
However, Thompson and Kelly (1957) and Villareal (1980) recommended another fertilizer program that can be applied to the soil for tomatoes. This program can be summarized as follows:
______________________________________ gm/m.sup.2 N P.sub.2 O.sub.5 K.sub.2 O Stages of growth 1* 2** 1 2 1 2 ______________________________________ Preplanting 5.7- 4.0 9.1- 15.0 6.8- 6.0 11.4 18.2 13.6 Growth 5.7- 4.0 -- -- -- -- 11.4 Fruiting 5.7- 4.0 -- -- -- -- 11.4 ______________________________________ *1 Thompson and Kelly; **2 Villareal
Similar to the greenhouse tomato, the cucumber requires a proper balance of nutrients to produce a good crop. Wittwer and Hornma (1979) suggested a weekly fertilizer schedule for greenhouse cucumber which can be represented as follows:
______________________________________ gm/m.sup.2 Stages of growth N P.sub.2 O.sub.5 K.sub.2 O ______________________________________ 1-3 weeks 4.8 3.4 7.5 4-10 weeks 29.0 6.8 21.1 11-18 weeks 47.7 8.0 24.7 ______________________________________
The highest yield of corn is obtained on heavily fertilized land. Usually, part of the nitrogen is applied in a mixed fertilizer during or before planting and a second application follows about 20 to 30 days after emergence of seedling. The suggested fertilizer program for corn is represented as follows:
______________________________________ gm/m.sup.2 Stages of growth N P.sub.2 O.sub.5 K.sub.2 O ______________________________________ Preplanting 11.36- 8.5 11.36 17.1 Growth 11.36- 8.5 11.36 17.1 ______________________________________
Beans are legume, and for many years only phosphorous and potash were used in fertilizer. Thompson and Kelly (1957) recommended to use the following fertilizer application for beans.
______________________________________ gm/m.sup.2 Stages of growth N P.sub.2 O.sub.5 K.sub.2 O ______________________________________ Preplanting 10.5- 66-99 27-54 21.0 Growth 10.5- 66-99 27-54 21.0 ______________________________________
Thompson and Kelly (1957) also reported a fertilizer program for eggplants which can be represented as follows:
______________________________________ gm/m.sup.2 Stages of growth N P.sub.2 O.sub.5 K.sub.2 O ______________________________________ Preplanting 11.4 17.1 17.1 Growth stage 2.3-4.6 -- -- ______________________________________
While the mulch film, and others disclosed in our copending applications, have been used with some degree of success, no plastic mulch film has been found which combines an adequate balance of the important properties needed and at the same time provides an optimal amount of nutrients to a crop during the various stages of plant growth.